Jeff Koons is king right now, as his much-talked-about retrospective at the Whitney Museum of American Art proves (945 Madison Ave.; whitney.org). But it’s New York’s Rockefeller Center, itself a kind of museum of decorative arts, that has the most visible of his works around: The larger-than-life Split-Rocker, a composite of two children’s rockers (one a pony, the other a dinosaur) covered in some 50,000 flowers (it has its own internal irrigation system), now sits in place of its annual Christmas tree. The massive work is a cousin of Koons’s Puppy piece, which occupied the same spot in front of 30 Rock in 2000 (the same year Split-Rocker made its debut in Avignon, France). Call it Rocker-feller center. Through September 12; between 49th and 50th Sts. and Fifth and Sixth Aves.; gagosian.com.